Catalytic cracking, notably fluidized catalytic cracking, FCC, is a well-established industrial process employed by the petroleum industry for converting high boiling hydrocarbon feedstocks to more valuable, lower average molecular weight, lower boiling hydrocarbon products useful as transportation fuels, notably gasoline. In the process, the conversion step is usually conducted by contacting the feedstock, e.g. a heavy gas oil, with a moving bed of particulate catalyst in the substantial absence of hydrogen at elevated temperatures.
The FCC process is cyclic and includes, for example, separate zones for conducting the catalytic reaction, steam stripping, and catalyst regeneration. In the cycle, feedstock is blended with the FCC catalyst in the catalytic reactor, generally referred to as a riser, wherein the conversion reaction is conducted. The lower boiling products are separated from the catalyst in a separator, suitably a cyclone separator, and the carbon deactivated catalyst is passed to a stripper and contacted with steam to remove entrained hydrocarbons; the latter being combined with vapors from the cyclone separator to form a mixture that is transported downstream for further treatment. The coked particulate catalyst is recovered from the stripper and passed to a regenerator, suitably a fluidized bed regenerator, and contacted with a combusting gas, e.g. air, at high temperature to burn off the coke and reactivate the catalyst. Regenerated catalyst is then blended with the feedstock entering the riser, this completing the cycle.
In the process, the particulate FCC catalyst is subjected to great mechanical stresses, unavoidably becoming attrited into smaller particles and escaping into the atmosphere. The escaping particulates not only pollute the atmosphere, but makeup catalyst must be added to the reactor. This, of course, is burdensome to the FCC operation. Accordingly, it is necessary that FCC catalysts be highly attrition-resistant.
FCC catalysts contain active crystalline aluminosilicate components such as zeolites, and active inorganic oxide components, notably clays of the kaolin type, dispersed within an inorganic oxide matrix formed from amorphous gels or sols which, on drying, bind the components together. It is desirable that the matrix also be active, attrition-resistant, selective with respect to the production of hydrocarbon liquids, and not readily deactivated by metals. Until recently the zeolite content of the FCC catalysts was low enough that the pore structure of the matrix was tailored to favor activity and selectivity over strength, or attrition resistance. However, present FCC catalysts contain high amounts of zeolitic material, above about 40 weight percent; sometimes as high as 60 weight percent, and greater. At these high concentrations of zeolite it is difficult to maintain a pore structure of high mesoporosity which is highly active and selective, while at the same time remaining highly attrition-resistant.
This is particularly so when it has become necessary, in order for refiners to maintain the profitability of their FCC units, to increase the feed rate, add higher molecular weight, lower quality feeds, and increase reactor temperature or the activity of the FCC catalysts, or both; conditions which can cause diffusional restraints that decrease the selectivity of the FCC catalysts to produce the desired high quality naphthas, as well as increase coke yield. Thus, high mesoporosity increases olefin and liquid yields, and lowers coke yields. However, increased mesoporosity is known to decrease the attrition-resistance of the catalyst. Thus, to affect these adverse consequences by increasing the mesoporosity of the FCC catalysts may appear desirable, but there is a trade-off. Various attempts have been made to balance these two apparently incompatible objectives, e.g. as summarized at Column 2, lines 1-57, and following, of my U.S. No. Pat. 4,968,405, which was issued on Nov. 6, 1990. Nonetheless, the problem is far from solved, and there remains a need for highly mesoporous highly attrition-resistant catalysts.